The present invention relates to a method of producing curable-rubber mixtures containing silica, for producing treads.
Here and hereinafter:
the term xe2x80x9csilicaxe2x80x9d is intended to mean a silicon dioxide-based reinforcing agent, silicates, and mixtures thereof;
the term xe2x80x9ccross-linkable unsaturated-chain polymer basexe2x80x9d is intended to mean any natural or synthetic noncross-linked polymer capable of assuming all the chemical, physical and mechanical characteristics typical of elastomers following cross-linking (curing) with sulfur-based systems;
the term xe2x80x9cmixture containing silicaxe2x80x9d is intended to mean a mixture containing reinforcing agents which definitely comprise silica, but which may also comprise, among other things, carbon black.
In the tire manufacturing industry, to minimize tire rolling resistance, carbon black, as a reinforcing filler, is replaced partly or fully by inorganic reinforcing fillers, such as chalk, talc, kaolin, bentonite, titanium dioxide, various types of silicate and, above all, silica.
Silica poses various drawbacks by being difficult to disperse in the polymer base, and, if dispersed poorly within the mixture, results in a mixture with highly variable, heterogeneous physical and mechanical characteristics.
The reason for this problem lies in the presence, on silica, of surface silanolic groups, which assist the formation of hydrogen bonds and, hence, silica particle clusters, and impart to the silica hydrophilic characteristics incompatible with the hydrophobic characteristics of rubber.
To solve the above dispersion problems, use has been made for some time now (see, for example, EP-B-0447066) of silane bonding agents, which prevent the formation of hydrogen bonds by bonding with silanolic groups, and, at the same time, chemically attach the silica to the polymer base.
The silica-silane bonding agent reaction is a critical passage in the preparation of mixtures containing silica, by requiring conformance with given temperature conditions. That is, the reaction must be conducted at a temperature of over 140xc2x0 C. to permit the reaction to take place rapidly, but below 175xc2x0 C. to prevent premature curing of the mixture. Moreover, the silica-silane bonding agent reaction produces a by-product of ethyl alcohol, which, on evaporating, is responsible, together with the water in the silica, for the formation of porous zones seriously impairing the dimensional stability of treads, so that a low tread strip drawing speed must be maintained to ensure the drawn product conforms with specifications.
In known methods of preparing mixtures containing silica, the silica-silane bonding agent reaction is made to occur after the polymer base has been mixed with other normally used ingredients. That is, known methods comprise a first mixing step, in which the polymer base is mixed with other ingredients; and a second mixing step, in which the mixture produced in the first step is mixed with silica and the silane bonding agent to bring about the silica-silane bonding agent reaction.
Alternatively, known methods provide for adding silica to the polymer base in the first step, and the silane bonding agent in the second step to bring about the silica-silane bonding agent reaction.
The above methods of preparing mixtures containing silica have several drawbacks, by failing to provide for correct silica dispersion, and by involving severe porosity initiating phenomena. That is, while successfully removing part of the water in the silica, they fail to remove the ethyl alcohol produced by the silica-silane bonding agent reaction, and, at the same time, have such low MOONEY scorch values as, at times, to impair the workability of the mixture.
Moreover, normally used methods can be extremely time-consuming when viewed in terms of output requirements and the number of production cycles performed on each production unit.
The present invention provides a method of preparing curable-rubber mixtures containing silica, for producing treadsxe2x80x94in particular, for passenger road vehicles, and which provides for ensuring correct silica dispersion in the polymer base and a substantial reduction in porosity by removing ethyl alcohol, as well as for higher MOONEY scorch values together with greater output.
According to the present invention, there is provided a method of preparing curable-rubber mixtures containing silica, for producing treads, the method being characterized by comprising at least a first step to obtain a first mixture by simultaneously loading into a mixer and mixing together at least a cross-linkable unsaturated-chain polymer base, a silica-based reinforcing filler, and a silane bonding agent; said first step being arrested upon said first mixture reaching a temperature ranging from 155xc2x0 C. to 175xc2x0 C.
The method defined above preferably comprises a second step to obtain a second mixture by adding to and dispersing in the first mixture at least one noncuring ingredient, said second step being arrested on reaching a temperature ranging from 145xc2x0 C. to 175xc2x0 C.; and a third step to obtain a third mixture by adding to and dispersing in the second mixture at least one curing agent, said third step being arrested at a temperature below a curing temperature of the third mixture.
Preferably, at least in said first step the ingredients are mixed together in one uninterrupted mixing comprising a first part, in which said mixer is operated at a relatively high first mixing speed, said first part terminating on reaching a temperature ranging from 120 to 145xc2x0 C.; and a second successive part, in which said mixer is operated at a second mixing speed lower than the first speed and such as to maintain the mixture at a temperature ranging from 150 to 165xc2x0 C.
Preferably, both said first step and said second step comprise said first part at said first mixing speed, and said second successive part at said second mixing speed.
Preferably, said first mixing speed ranges from 50 to 80 rpm, whereas said second mixing speed ranges from 20 to 50 rpm; the difference between said two mixing speeds being at least 10 rpm.
In the method defined above, part of the silica is preferably also added in the second step.
The amount of silica added in the first step preferably constitutes 50% to 75% by weight of the total amount of silica used.
The method set forth above preferably comprises, between the second and third step, a further remixing step, in which no ingredients are added, and which is stopped on reaching a temperature ranging from 135xc2x0 C. to 175xc2x0 C.
Preferably, said further remixing step comprises said first part at said first mixing speed, and said second successive part at said second mixing speed.
In the method defined above, a carbon black-based reinforcing filler is preferably added in the first step.
Further embodiments are identified in the Examples.